Standing allows full control over your body, its tension and balance. Sitting cuts off the lower half which might feel like more stabilized at the first moment, but, especially when playing instruments with a wide pitch field (tVox tour or Etherwave with ESPE01), tuned for a reasonably linear and wide tone spacing, you need to move within the field and you'll feel soon disabled when you try to do that sitting.

Don't forget, the Theremin is only a box full with electronic components. The true music instrument in that configuration is the player himself who acts with its whole body on two fields.

Take the example of Clara Rockmore, who at age 80+ played still standing like a soldier with full control over each nerve and muscle of her body. If she could do that, younger people have really no reason to sit.

This applies to "classic" theremin configurations with a vertical pitch rod and a horizontal volume antenna. Other instruments with plate antennas will behave differently. I have no experience with the latter, so I can't advise for these.

Thierry, a comfortable 8 octave range between my hand on my sternum and the arc bending from the elbow down to the antenna, with the top two octaves addressed by fine motor hand motions aiming or the center of the plate with no need for vibrato except for the top two octaves which tend to be a tad dicey because, hey, i'm 64.